This invention relates to a vehicle skid control system and, more particularly, to a "pumping-type" or "pulse action" vehicle brake control system that advantageously utilizes a novel frequency converter, a novel ramp generator, a novel pulse-width detector, and a novel automatically adjustable switching circuit.
When braking an automobile under emergency stopping conditions or under adverse road conditions, the possibility exists that the vehicle will go into an uncontrollable skid or a controllable skid which prevents the driver from bringing his vehicle to a safe stop within the distance available. In either case, one factor that indicates an imminent skid is the deceleration rate of the rear wheels. Some improvement in vehicle stability can be achieved by automatically "pumping" or pulsing the brakes at the rear wheels in an arbitrary pre-programmed way.
Recently, a system has been developed that operates on the principle of inhibiting the normal braking action initiated by the automobile operator. In this system, wheel speed sensors generate signals that are proportional to wheel speed. The wheel speed signals are processed through a control module which generates a voltage to energize a solenoid in an actuator that controls the hydraulic braking system to the rear wheels. When a skid is imminent, a command signal from the control module causes the actuator solenoid to close a vacuum passage in a diaphragm chamber. By action of the diaphragm and the normal hydraulic pressure from the master cylinder as developed by the automobile operator, the hydraulic pressure to the rear wheels is released, thereby inhibiting the rear wheel braking action. When the rear wheels spin-up to the vehicle speed, the control module produces a signal to de-energize the actuator solenoid. This restores line pressure and reapplies the rear brakes. In effect, this system "pumps" the rear brakes in a manner often recommended for controlled stopping under adverse driving conditions.
The control module includes a frequency converter for each wheel speed sensor to convert a frequency varying signal into a direct current signal. A summation in a summing amplifier of the frequency converter outputs produces a composite of the wheel speed signals. A deceleration rate detector and an acceleration rate detector respond to the output of the summing amplifier to produce outputs proportional to the rate of deceleration and the rate of acceleration, respectively, of the rear wheels of a motor vehicle. A signal proportional to the output of the summing amplifier is transferred to a vehicle velocity ramp generator and one input of an automatically adjustable switching circuit. The velocity ramp generator produces a step ramp function having an overall slope related to the actual speed of the automobile when braking to a stop. In addition to a signal related to wheel speed, the switching circuit also has an input from the vehicle velocity ramp generator and an input from a retarding force detector. An output is produced at the switching circuit whenever the summation of a wheel speed signal, a velocity ramp signal, and a retarding force signal reaches a threshold condition. The retarding force detector produces an output signal which relates to the braking factors including tire condition, brake condition, and the condition of the road surface. To generate a control signal to the actuator solenoid, the output of the deceleration rate detector, the acceleration rate detector, and the switching circuit must have a certain designated relationship. These three signals are the inputs to a brake controller as the last component in the control module.
An object of the present invention is to provide a control module for processing wheel speed signals in a vehicle skid control braking system.
Another object of this invention is to provide a control module for producing a pumping type braking action in a skid control vehicle braking system.
A further object of this invention is to provide a control module for generating brake inhibit signals in a vehicle skid control braking system.
Still another object of this invention is to provide a control module for generating brake inhibit signals at decreasing vehicle speed in a vehicle skid control braking system.
Still another object of this invention is to provide a control module for processing wheel speed signals in a vehicle skid control braking system that advantageously utilizes a frequency converter, a ramp generator, a pulse-width detector and an automatically adjustable switching circuit.
In accordance with the present invention, a wheel sensor, which may be coupled to the rear wheels of an automobile or to the drive shaft of an automobile, generates signals having a frequency varying in accordance with wheel speed. These frequency varying signals are connected to inputs of frequency converters that produce a D.C. output voltage having a magnitude that varies in accordance with the frequency of the wheel sensor signals. The outputs of the frequency converters are averaged and coupled to the input terminals of a deceleration rate detector and an acceleration rate detector. The deceleration rate detector responds to the averaged signal and generates a control signal output at a brake inhibit level when the rate of deceleration of the rear wheels exceeds a preset limit. The acceleration rate detector also responds to the averaged signal varying in accordance with wheel speed to generate an output at a brake inhibit level whenever the rate of rear wheel acceleration does not exceed a preset limit. An automatically adjustable switching circuit also responds to the wheel speed voltage to clamp the acceleration rate detector output at a normal braking level until the wheel speed drops below a value varying in accordance with vehicle speed and braking conditions. The output of the deceleration rate detector, the acceleration rate detector, and switching circuit are combined to form the inputs to a brake controller that energizes an actuator solenoid whenever the outputs of both rate detectors are at a brake inhibit level. A comprehesive embodiment of this invention includes a frequency converter, a ramp generator, a pulse width detector, and an automatically adjustable switching circuit. The frequency converter receives signals from the sensor which has a frequency proportional to wheel speed. This frequency changing signal is amplified in a first amplification stage and connected to a second amplification stage for producing a square wave output. The square wave output produced by the second amplification stage is transferred to an inverter amplifier which produces a second square wave having a 180.degree. phase shift from the first square wave. The leading edge of the two square waves are individually differentiated and the differentiated pulses connected to an integrator. After processing by the integrator, the resulting signals are combined to produce a wave having an average value related to wheel speed. The ramp generator produces an output current that varies inversely with vehicle speed. A voltage proportional to vehicle wheel speed charges an information storage circuit during normal running operation. In a braking mode, the information storage circuit is discharged at a controlled rate so long as normal braking action takes place. When a brake controller inhibits the normal braking operation, the charge in the information storage circuit is held at a fixed level. The controlled discharge begins again from the previous fixed level when the normal braking operation is resumed. The composite discharge of the information storage circuit is along a ramp that varies with vehicle speed. The pulse width detector generates a current waveform that varies with braking factors including road surface condition. A brake controller provides a variable width square wave pulse train to the input of the pulse width detector that includes a source of constant current to charge an information storing circuit. Charging of the information storing circuit takes place only when the brake controller inhibits the normal braking function. At the beginning of the inhibit cycle, a reset circuit discharges the information storage circuit in response to a reset pulse produced by differentiating the leading edge of the input pulses generated at the brake controller. A current amplifier connected to the information storage circuit produces a current signal to the speed inhibit switch that varies in accordance with pulse width of the brake controller. The automatically adjustable switching circuit controls the output of the acceleration rate detector. A signal proportional to vehicle wheel speed, a signal varying in accordance with braking control factors, such as road surface characteristics, and a ramp current inversely related to vehicle speed are connected to inputs of the switching circuit. When the sum of these three signals reaches a threshold condition, the switching circuit operates to release the output of the acceleration rate detector that is then applied as one input to a brake controller. Since both the braking control factor signal and the ramp current are variable signals, each time the switching circuit operates, it releases the acceleration rate detector at a different wheel speed in the operation of a skid control system.
A more complete understanding of the invention and its advantages will be apparent from the specification and claims and form the accompanying drawings illustrative of the invention.